This invention relates to a process of resin transfer molding lightweight, foam-filled products and the strong, lightweight products made thereby. More particularly, the present invention is directed to a process of resin transfer molding a wind tunnel blade and the structure of the wind tunnel blade.
Resin transfer molding has been around for many decades, and its use has grown considerably in recent years. The process allows the economical manufacture of high quality composites. In accordance with the process, a resin system is transferred at low viscosities and low pressures into a closed mold die containing a preform of dry fibers. The dry fibers, which may have the form of continuous strand mat, unidirectional, woven, or knitted preforms, are placed in a closed mold and resin is introduced into the mold under external pressure or vacuum. The resin cures under the action of its own exotherm, or heat can be applied to the mold to complete the curing process.
The resin transfer molding process can be used to produce low-cost composite parts that are complex in shape. These parts typically provide continuous fiber reinforcement, along with inside mold line and outside mold line controlled surfaces. It is the placement of the continuous fiber reinforcements in large structures that sets resin transfer molding apart from other liquid molding processes.
In the past, resin transfer molding was used for applications suitable to consumer product markets. However, in the last few years, through the development of high-strength resin systems and more advanced pumping systems, resin transfer molding has advanced to new levels. These recent developments have promoted resin transfer molding technology as a practical manufacturing option for high-strength composite designs, particularly in the aerospace industry.
In the aerospace industry, the most visible advantage to the resin transfer molding process lies in resin transfer molding""s ability to combine multiple, detailed components into one configuration. For example, many traditional designs consist of many individual details that are combined as a subassembly. These subassemblies usually require labor-intensive shimming, bonding, mechanical fastening, and sealing. Consequently, these subassemblies demonstrate high part-to-part variability due to tolerance build-up.
Resin transfer molding produces an aerodynamic, decorative finish, with controlled fit-up surfaces. Being a product of the mold makes the surface quality of the part produced within the mold comparable to that of the tool""s surface.
Resin transfer molding also provides control of the fiber/resin ratio in the completed product. This advantage produces parts that are lightweight and high in strength.
Unlike conventional composite systems that use lay-up of prepreg materials, resin transfer molding does not require an autoclave. Therefore, no autoclave costs are incurred, no size limitations are inherent, and no staging issues occur.
In terms of raw material cost, resin transfer molding offers cost savings by using bulk materials like broad goods. Because dry goods are less expensive than preimpregnated materials, savings can be associated with the cost of the wasted material during the ply-knitting operation. Also, bulk materials do not require special handling requirements such as freezer storage.
The basic injection operation of resin transfer molding is straightforward and easily learned. Hence, minimal training is required to bring operators on line. On the other hand, in making preforms, the level of operator skill and training is dependent upon the method of preforming that is used. Preform fabrication methods include braiding, knitting, weaving, filament winding, and stitching. Each of these methods is quite different and must be individually evaluated for specific design characteristics.
The initial capital investment costs of resin transfer molding are low when compared with many other molding processes. An elementary form of resin transfer molding can be achieved using a pressure pot, an oven, and a vacuum source. A variety of commercially available equipment can be used to enhance the process in many areas.
In most cases, resin transfer molded materials can be formed with minimal chemical exposure to workers and their environment. Many high-performance resin systems are stable and release low volatiles. Since resin transfer molding is processed within a closed system, workers are exposed to the resin only when loading the dispensing equipment.
One of the problems encountered when using resin transfer molding is that complex cavities that extend into the surface of the part must be formed in the mold cavity surface, or the complex cavity will be filled by resin during the resin injection process. If the complex cavity is designed to receive a bushing or an insert, the bushing or insert can be incorporated into the preform and injected in place to eliminate some higher level assembly and to avoid the need for a complex tooling surface. If the part includes an internal hollow tube, proper design of the tool to take this into account may be difficult and expensive, or may produce a tooling configuration from which removal of the finished part would be difficult.
Other problems are encountered in laying up or arranging preforms of fibers prior to placing the preform into the mold. If braided or woven fabric is used, cutting of that fabric often results in frayed edges, which is undesirable. Arranging stacks, or tapered-off sections of the preforms on a substrate so that ply drops are aligned correctly is also difficult.
The present invention solves many of the above problems by providing a series of unique processes for the fabrication of a wind tunnel blade. The processes result in a new structure for a wind tunnel blade.
It has become conventional practice in the aircraft industry to manufacture helicopter and other blades having a molded fiber-reinforced resin body formed by resin transfer molding. The fiber-reinforced resin bodies were often formed about an internal, metallic, load-bearing spar. Such fiber-reinforced resin bodies exhibited high strength and low weight characteristics. With the exception of the internal metal spar, however, prior art resin transfer molded rotor blades did not include structural reinforcements along their length.
Prior art wind tunnel blades were formed from a lay-up of prepreg composite material that was shaped into a unitary structure including a base attached to the blade. The housing and the hub for the wind tunnel blades required that a technician lay on his back and install the unitary base and blade structure into the wind tunnel""s hub, which was difficult.
Because prior art wind tunnel blades were subjected to high speed wind conditions, the wind tunnel blades were often damaged as a result of fatigue and wind erosion. To counter this wind erosion, the prior art wind tunnel blades included frangible foam tips at their distal ends. The frangible foam tips were often formed of a foam material having a uniform density. The frangible foam tip was wrapped in plies of fiberglass to protect the foam from wind erosion and to improve impact resistance. This wrapped fiber piece was difficult to form, and required a large amount of labor to produce.
Prior art wind tunnel blades were difficult to balance because the wind tunnel blades were not of uniform weight and did not have consistent centers of gravity. The prior art wind tunnel blades were balanced by adding lead weights to the blade butt to adjust the center of gravity. After the center of gravity was adjusted, the blade must be matched to another blade of approximately the same weight. This matching process can be difficult because of the large blade-to-blade variation in weight.
The present invention solves the above problems by providing a novel wind tunnel blade design incorporating a variety of different features that permit easier installation, service, and replacement of the wind tunnel blades. The process of forming the unique wind tunnel blade incorporates a number of new composites forming techniques. These techniques are applicable to a number of parts or products, and can be used to form parts having a number of different configurations or complex shapes.
The present invention provides a plug including a flexible outer bushing having first and second ends, a connector attached to the first end of the bushing, and a fastener extending along the flexible outer bushing and attached to the connector. The fastener is configured such that actuation of the fastener causes the flexible outer bushing to expand outward, whereby the flexible outer bushing can be inserted into a hollow opening and can expand against the sides of the opening by actuation of the fastener.
In one embodiment, the connector is a female-threaded insert. The fastener can extend along the bushing and includes (1) an abutment surface for engaging the second end of the bushing and (2) male threads that are received in the female-threaded insert. Actuation of the fastener involves rotating the fastener to move the connector towards the second end.
In accordance with another aspect of the plug, the fastener extends along the bushing and comprises an abutment surface for engaging the second end of the bushing and actuation of the fastener comprises causing the fastener to pull the connector toward the abutment surface.
The present invention also provides a method of resin transfer molding a product having a hollow tube therein. The method includes placing an expandable plug into a hollow tube so that a portion of the plug extends along the intended finished line of the product being formed, and expanding the expandable plug so that the expandable plug is pressed against the outer sides of the hollow tube. Resin is injected about the hollow tube and around the plug in a resin transfer molding process such that excess resin is formed beyond the intended finish line. The excess resin and the expandable plug are cut along the intended finish line so that the plug is no longer expanded and falls out of the hollow tube.
The present invention further provides a reinforced core structure for use in a resin transfer molding process. The reinforced core structure includes an expanded core having a longitudinal axis, a first set of braided fibers extending from a first end of the expanded core to a first location and reversing from the first groove over itself and back towards the first end, and a second set of braided fibers extending from the first end over the first set of braided fibers and to a second location beyond the first location and reversing from the second location, back over itself and rearward to the first end.
In one embodiment, the expanded core includes a plurality of grooves extending transverse to the longitudinal axis.
In accordance with another aspect of the invention, a first groove is located at the first location, and a first cord ties off the first set of braided fibers and extends between the overlapped layers of the first set of braided fibers and opposite the first groove so that the first cord presses the first set of braided fibers into the first groove. A second groove can be provided that is located at the second location. A second cord ties off the second set of braided fibers and extending between the overlapped layers of the second set of braided fibers and opposite the second groove so that the second cord presses the second set of braided fibers into the second groove.
Preferably, the perimeter of the expanded core between the first and second grooves is substantially the same as the perimeter of the expanded core in the region between the first groove and the end and the overlapped layers of the first set of braided fibers extending over this latter area.
A third set of braided fibers can be provided that extends from the first end, past the first and second grooves, to a third groove beyond the second groove and reversing at the third groove over itself and back to the first end. A third cord can be provided that ties off the third set of braided fibers and extends between the overlapped layers of the third set of braided fibers and opposite the third groove so that the third cord presses the third set of braided fibers into the third groove.
Preferably, the perimeter of the expanded core between the first and second grooves and the overlapped layers of the second set of braided fibers extending thereover is substantially the same as the perimeter of the expanded core in the region between the second and third grooves.
The present invention further provides a method of forming a reinforced core structure for use in a resin transfer molding process. The method includes providing an expanded core having a longitudinal axis, braiding a first set of fibers from a first end of the expanded core to a first location on the expanded core, and reversing the direction of the braiding of the first set of fibers at the first location and continuing braiding back to the first end so that the first set of braided fibers is braided back upon itself to form a first dual layer fiber structure. A second set of fibers is braided over the first set of braided fibers from the first end beyond the first location to a second location. The braiding direction of the second set of fibers is reversed at the second location back toward the first end so that the second set of braided fibers is braided back upon itself to form a second dual layer fiber structure.
In accordance with one aspect of the method, the first set of braided fibers are tied at the first location with a cord before reversing direction of the braided fibers. The second set of braided fibers are tied at the second location with a cord before reversing direction of the braided fibers.
The expanded core can be provided with a plurality of grooves extending transverse to the longitudinal axis. A first groove is located at the first location, and the first set of braided fibers is tied with a cord before reversing direction of the first set of braided fibers. The cord is arranged opposite the first groove such as to pull the first set of braided fibers into the first groove. A second groove is located at the second location, and the second set of braided fibers is tied with a cord before reversing direction of the second set of braided fibers. The cord is arranged opposite the groove such as to pull the second set of braided fibers into the second groove.
The method further provides braiding a third set of fibers from the first end over the first and second sets of braided fibers to beyond the second groove to a third groove and reversing the braiding direction of the third set of fibers at the third groove back toward the first end so that the third set of braided fibers is braided back upon itself to form a third dual layer fiber structure.
In accordance with another aspect of the present invention, a method of preparing a reinforced core structure for a product to be formed in a resin transfer molding process utilizing a resin is provided. The method includes applying fibers over a core beyond the final finished line for the product to be formed, applying a tackifier solution to the fibers located at the final finish line, the tackifier solution comprising a reduced resin concentration from the final resin concentration of the product to be formed in the resin transfer molding process, locally consolidating the tackifier solution, and cutting along the final finish line.
Preferably, the tackifier solution includes resin to be used for the resin transfer molding process diluted by a solvent.
The present invention further provides a radius filler for use in a resin transfer molding system. The radius filler includes unidirectional tows and a braided sleeve of fibers extending around the unidirectional tows. A tackifier solution can be added to the braided sleeve, the tackifier solution comprising a diluted mixture of the resin to be used in the resin transfer molding system. The tackifier solution can include resin to be used for the resin transfer molding process diluted by a solvent.
The present invention further provides a method of forming a radius filler for use in forming a preform to be used in a resin transfer molding process, the method including providing unidirectional tows, and braiding a sleeve of fibers around the unidirectional tows. A tackifier can be applied to the braided sleeve, the tackifier including a diluted solution including the resin to be used in the final resin transfer molding process. The tackifier is consolidated so as to lend rigidity to the radius filler.
The present invention further provides a method of forming a core structure including providing a mold having an internal cavity, arranging a prepreg along the inside of the internal cavity, the prepreg being of a size such that the prepreg can extend around a circumference of the mold, placing an expandable foam material in the cavity of the mold and within the prepreg material, heating the expandable foam material so as to expand the foam material within the prepreg material so to press the prepreg material against the walls of the cavity of the mold, and curing the expandable foam material and the prepreg material so as to form the core structure.